husafreak,
I have a YS 110S that had a similar problem when it was brand new. It would hammer violently and loudly and then stop suddenly and slip the prop.
No amount of adjusting the low or high-speed needle valves or the regulator had any corrective effect on this condition. There was some kind of intermittent blockage in the fuel system that may have come with the engine from the factory.
If you go to the YS Engines Support thread at
http://www.rcuniverse.com/forum/m_7866253/tm.htm you can read the suggestions that were offered and the actions that I took solve the problem.